A disk brake of motorcycles, automobiles, and bicycles inhibits rotation of a wheel by friction between a brake disk and a brake pad and thereby controls braking of a vehicle. Therefore, the brake disk is required to have an appropriate hardness. If the hardness is insufficient, the brake is less effective and the brake disk is rapidly worn by the friction with the brake pad. On the other hand, if the hardness is too high, there is a problem that brake squeal is apt to occur. The recommended appropriate hardness of a brake disk is in the range of about 32 to about 38 HRC. Here, the term HRC means Rockwell hardness (C-scale) determined in accordance with the specifications of JISZ2245.
Martensitic stainless steel sheets have been conventionally used as a brake disk material from the viewpoints of hardness and corrosion resistance. In the past, high-carbon martensitic stainless steel sheets, such as SUS420J2, were used after quenching and tempering. However, since the amount of work required for manufacturing was large, their use was temporary. Recently, low-carbon martensitic stainless steel sheets, which only require quenching as disclosed in Japanese Unexamined Patent Application Publication No. 57-198249 and Japanese Unexamined Patent Application Publication No. 60-106951, have been widely used as a brake disk material.
Recently, an improvement in the fuel efficiency of motorcycles and automobiles has been desired from the viewpoint of conservation of the global environment. Fuel efficiency can be effectively improved by reducing vehicle weight. Therefore, many studies are now aimed at vehicle weight reduction. Disk brakes, which are a braking device, are not an exception. Hence, the brake disk is miniaturized and the thickness is decreased (thin wall) in order to further reduce vehicle weight. However, the miniaturization and thin-walling of brake disks induce a decrease in heat capacity and, thereby, the degree of increase in brake disk temperature due to friction heat by braking is increased. Consequently, the brake disk temperature during braking has increased to about 600° C. with such miniaturizing and thin-walling. Therefore, it is a concern that brake disks formed of conventional materials are softened by tempering and that the durability and corrosion resistance thereof are decreased. Hence, materials with excellent heat and corrosion resistances for brake disks are desired.
To satisfy such a desire, for example, Japanese Unexamined Patent Application Publication No. 2002-146489 discloses a low-carbon martensitic stainless steel sheet containing at least one of Ti, Nb, V, and Zr in an appropriate amount. The softening due to an increase in temperature during use of a disk brake is effectively suppressed and, thereby, a decrease in hardness can be suppressed. In addition, Japanese Patent No. 3315974 discloses a stainless steel containing Nb or further containing a combination of Ti, V, and B in an appropriate amount in addition to Nb, which can effectively suppress temper-softening and is used for a brake disk.
Furthermore, Japanese Unexamined Patent Application Publication No. 2002-121656 discloses an inexpensive steel which is used for a rotor of a disk brake. The deterioration in material quality due to an increase in temperature during use of the disk brake can be substantially prevented by adjusting a GP value to about 50% or more and adjusting the amounts of Nb and V to appropriate values. The GP value is a relational expression of C, N, Ni, Cu, Mn, Cr, Si, Mo, V, Ti, and Al contents in the steel. In Japanese Unexamined Patent Application Publication No. 2001-192779, a martensitic stainless steel with high resistance against softening due to heat by braking is disclosed for use in brake disks. The steel can retain a desired hardness and have high resistance against temper-softening after conducting only quenching by controlling the total amount of C and N within a specific range and containing Mn, Ni, and Cu in appropriate amounts. Mn, Ni, and Cu are austenite-forming elements. The steel further contains Nb in an appropriate amount and at least one of Zr, Ti, and Ta.
However, in the technologies disclosed in Japanese Unexamined Patent Application Publication No. 2002-146489, Japanese Patent No. 3315974, Japanese Unexamined Patent Application Publication No. 2002-121656 and Japanese Unexamined Patent Application Publication No. 2001-192779, a relatively large amount of expensive alloy elements are necessarily used. Therefore, the manufacturing costs of brake disks are excessively high. Furthermore, the hardness is rapidly decreased and, simultaneously, corrosion resistance is decreased when the brake disks are maintained at about 600° C. for a long time (about 2 hours). Thus, there are problems.
It could therefore be advantageous to provide a stainless steel sheet with excellent heat and corrosion resistances for a brake disk. The stainless steel sheet should retain an appropriate hardness after quenching and can maintain a hardness of about 32 HRC or more (Rockwell hardness C-scale) determined in accordance with the specifications of JISZ2245 and have only a slight decrease in corrosion resistance after holding at about 600° C. for about 2 hours.